The present invention relates to a method and a device for aiding the monitoring of a turbine engine of an aircraft, in particular of a transport aeroplane, while said turbine engine is in a sub-idle operating phase.
“Sub-idle operation” relates to an operating state of the turbine engine, particularly during the start phase or during recovery after an ill-timed dip below the lowest power ensuring stable operation, for which the turbine engine rotates more slowly than when it is idling.
Generally, to monitor the start or reacceleration towards idle speed of the engines of an aircraft, the aircraft crew is obliged to check several digital and/or analogue parameters simultaneously, which are displayed on various screens or instruments (particularly on a main ECAM page and a secondary engine page). In particular, the crew have to monitor the following in sequence and/or in parallel:                the opening of the start valve;        the establishment of the correct air pressure required for start;        the gradual increase in the speed of the high-pressure (HP) module of the turbine engine, which is monitored using a parameter N2 or N3 which indicates the rotational speed of the HP shaft. For convenience, the speed of the HP module will be denoted generally as N2 hereafter; however, either N2 or N3 is meant indiscriminately, depending on the body number of the turbine engine in question;        the opening of the HP fuel tap when the HP shaft is rotating at the right speed;        the actuation of the igniter plug(s);        the establishment of the correct mass fuel rate of flow for start;        ignition, which is identifiable by the increase in the gas outlet temperature, which is monitored using a parameter EGT (or “exhaust gas temperature”) or TGT (or “turbine gas temperature”). For convenience, the gas outlet temperature will be denoted generally as EGT hereafter; however, either EGT or TGT is meant indiscriminately; and        the gradual and steady increase in the gas outlet temperature EGT in line with the simultaneous increase in the rotational speed of the HP module, while ensuring that these parameters, and particularly the gas outlet temperature EGT, remain below maximum values set by the engine manufacturer and that the rotational speed of the HP shaft does not stagnate.        
To monitor the gas outlet temperature (EGT), most engine manufacturers take into account only a set limit, generally shown in red, which is published in the operating manuals and thus known to the aircraft crew. However, some engine manufacturers add to this a limit which can vary as a function of the speed N2 of the HP module and is generally shown in amber. As a criterion, this temperature limit depending on the rotational speed is closer to the physical reality of the turbine engine; however, the crew cannot memorise it since it changes (and is therefore not published in the operating manuals). This variable limit is taken into account by the warning logics of the turbine engine control unit, but the crew has no means of easily anticipating the sounding of an overheating warning or a start failure and generally simply has to react if the need arises.
In particular, FR-2 821 452 discloses an electronic centralised aircraft monitoring (or ECAM) device which automatically monitors a plurality of systems, including the engines of an aircraft, and informs the aircraft crew of the state of these systems. In a particular embodiment, this monitoring device displays, among other things, the gas outlet temperature EGT and the rotational speed N2 of the high-pressure module of the engines on different screens or different pages.
Furthermore, when in flight and with conventional display means, the distinction between an engine in standard operation and an engine in sub-idle operation is not immediately obvious, instead requiring the careful reading of a number of other particular parameters (in particular the N2 or N3 parameter).